build: split EXTRA_DIST files in src/ off from DISTFILES and handle separately

This is again to avoid the "Argument list too long" error we are hitting more and
more now. Given we just merged elementary, emotion generic players, evas generic
loaders and elm_code it is not surprising we are hitting it again.
This time the number of files being hold in DISTFILES has just grown to big so a
make dist was no longer possible. If one looks at what the DISTFILES variable
from automake holds you can image it grows a lot with all the source files plus
generated files we have in tree now.
DISTFILES = $(DIST_COMMON) $(DIST_SOURCES) $(TEXINFOS) $(EXTRA_DIST)
To cut off a big chunk but still keep all the other automagic in place for
SOURCE files I went and renamed the EXTRA_DIST in src/ to EXTRA_DIST2 and handle
the files in a dist-hook now.
Another thing to note here is that this also only happens as we have the one big
Makefile with includes. If we go back to per directory Makefiles this problem
should vanish as well. In any case we need a solution for 1.18 now and this is
what I have to offer. If you have a cleaner solution in mind feel welcome to
test it out and if everything we need keeps working (make, make examples,
make check, make benchmark, make dist and make distcheck) go ahead.